With the development of the electrical bicycle (EB), there are more and more requirements for the collaboration between a battery management system (BMS) and a charger of the EB. The BMS monitors the state of a battery pack. The charger charges the battery pack according to the state of the battery pack. Digital signals are transferred between the BMS and the charger. A voltage of the digital signal is relatively low, e.g. 3 V, when the digital signal is logic high. A voltage difference between a logic high level of the digital signal and a logic low level of the digital signal is relatively small. There is usually a large amount of electromagnetic interference around the EB. Thus, the bit error rate of the digital signal may be relatively high.